Drive transmission device and image forming apparatus

ABSTRACT

A drive transmission device includes a first gear, a cam configured to rotate in interrelation with rotation of the first gear and capable of changing a state of two members between a pressed state and a pressure-released state, a pressing member configured to press the cam, and a second gear configured to transmit drive to the first gear in engagement with the first gear. At least one of the first and second gears includes first teeth provided at positions corresponding to an engaging region in which the first gear is engaged with the second gear when the state of the two members is changed from the pressure-released state to the pressed state and includes second teeth provided at positions corresponding to a region other than the engaging region. Tooth thicknesses of the first teeth are thicker than tooth thicknesses of the second teeth.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional application of U.S. patent applicationSer. No. 15/976,186, filed May 10, 2018, which claims the benefit ofJapanese Patent Application Nos. 2017-095484, filed May 12, 2017, and2018-027517, filed Feb. 20, 2018, the entire disclosures of which areall hereby incorporated by reference herein

FIELD OF THE INVENTION AND RELATED ART

The present invention relates to a drive transmission device and animage forming apparatus, such as a copying machine or a printer,including the drive transmission device.

In the image forming apparatus such as the copying machine or theprinter, various mounting and demounting units detachably mountable toan apparatus main assembly are provided in order to improve operativityand a maintenance property. Also a fixing unit for fixing a developerimage on a recording material and an intermediary transfer unit forprimary transferring the developer image formed on a surface of aphotosensitive drum are provided detachably mountable to the apparatusmain assembly.

A load of several N to several tens of N is exerted on a press contactnip such as a fixing nip of the fixing unit or a primary transfer nip ofthe intermediary transfer unit. For this reason, when these units areleft standing for a long term in a pressed state, permanent deformationgenerates in an elastic layer at the press contact nip, so that evenwhen the load at the press contact nip is released (eliminated), thedeformation of the elastic layer is not returned to an original state.As a drive transmission device for stably and reliably transmittingdrive (driving force) to the mounting and demounting unit detachablymountable to the apparatus main assembly, a coupling, a swingable gearor the like is used.

Japanese Laid Open Patent Application (JP A) 2005 157112 discloses thata driving source is reversely rotated depending on an amount of play ofa coupling, an amount of backlash of a driving system or an amount ofrotation until the coupling is disconnected (disengaged) with respect toan axial direction. As a result, a resistance of the coupling during aspacing (separating) operation can be alleviated. In JP A 2013 045026, adrive transmission device in which a driving side gear is provided issupported by a frame member in a cantilever state and is providedswingably along an axial center direction of the driving side gear witha supporting portion thereof as a fulcrum, so that the driving side gearengages with a driven side gear. As a result, a flexible property of thedriving side gear is ensured, so that engagement of the gears becomessmooth against a mounting error and a load fluctuation.

In JP-A 2015-087701, a pressure of a pressure generating member isadjusted by changing a rotation angle of a cam. When two outerperipheral points of the cam contact a first member and a second memberby rotation of the cam, a rotational force of the cam is cancelled bythe pressure of the pressure generating member. As a result, generationof impact noise during pressure application is prevented.

Even when the load at the press contact nip such as the fixing nip ofthe fixing unit or the primary transfer nip of the intermediary transferunit is released, the deformation of the elastic layer is not returnedto the original state. In order to prevent this phenomenon, a pressurechanging device using a pressure releasing cam is provided forautomatically eliminating a press contact state at the press contact nipin a period other than during image formation. When the drivetransmission is carried out so that a state of the press contact nip ischanged from a pressed state to a pressure released state in themounting and demounting unit detachably mountable to the apparatus mainassembly, a force generates in a direction of drawing the mounting anddemounting unit to the apparatus main assembly, and therefore, the drivetransmitting gears reliably engage with each other.

On the other hand, when the drive transmission is carried out so thatthe state of the press-contact nip is changed from the pressure-releasedstate to the pressed state, the pressure-releasing cam is rotated inadvance by the pressure from the pressure changing device. For thisreason, a force for outwardly moving the mounting and demountingrelative to the apparatus main assembly generates, so that the impactnoise during the pressure application becomes large due to play of thedrive transmitting gears. At this time, in order to reliably receive arotational force of the pressure-releasing cam rotating in advance, anincrease in rigidity of the mounting and demounting unit is required,and therefore, led to increases in size and cost. Further, there is alsoa liability that when the mounting and demounting unit is mounted in theapparatus main assembly, tooth tops of the drive transmitting gears abutagainst each other and thus a mounting operation is obstructed.

SUMMARY OF THE INVENTION

The present invention has solved the above-described problem. Aprincipal object of the present invention is to provide a drivetransmission device in which an engaging performance of gears fortransmitting drive (driving force) to a cam capable of changing apressed state between two members is improved.

According to an aspect of the present invention, there is provided adrive transmission device comprising: a first gear; a cam configured torotate in interrelation with rotation of the first gear and capable ofchanging a state of two members between a pressed state and apressure-released state; a pressing member configured to press the cam;and a second gear configured to transmit drive to the first gear inengagement with the first gear, wherein at least one of the first gearand the second gear includes a plurality of first teeth provided atpositions corresponding to an engaging region in which the first gear isengaged with the second gear when the state of the two members ischanged from the pressure-released state to the pressed state andincludes a plurality of second teeth provided at positions correspondingto a region other than the engaging region, and wherein tooththicknesses of the first teeth are thicker than tooth thicknesses of thesecond teeth.

Further features of the present invention will become apparent from thefollowing description of exemplary embodiments with reference to theattached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a sectional illustration showing a structure of an imageforming apparatus.

FIG. 2 is a perspective illustration showing a mounting and demountingdirection of a fixing unit.

FIG. 3 is a sectional illustration showing a structure of a fixing unit.

FIG. 4 is a perspective illustration showing a structure of a heatingunit and a pressing roller which are provided in the fixing unit.

FIG. 5 is a sectional illustration showing the structure of the heatingunit and the pressing roller which are provided in the fixing unit.

FIG. 6 is a perspective illustration showing a structure of a pressingmember for applying pressure to the heating unit and the pressing rollerwhich are provided in the fixing unit.

FIG. 7 is a sectional illustration of the fixing unit during a normaloperation in which a fixing nip is formed by the heating unit and thepressing roller which are provided in the fixing unit.

FIG. 8 is a sectional illustration showing a state in which the fixingnip between the heating unit and the pressing roller which are providedin the fixing unit is eliminated.

FIG. 9 is a perspective illustration showing a structure of a drivetransmission device for transmitting a rotational driving force to a cammember.

Part (a) of FIG. 10 is a side illustration showing an engaging statebetween transmission gears in a pressed state between the heating unitand the pressing roller, and part (b) of FIG. 10 is a side illustrationshowing a state of the cam member in the pressed state between theheating unit and the pressing roller.

Part (a) of FIG. 11 is a side illustration showing an engaging statebetween the transmission gears during a change of a state between theheating unit and the pressing roller from the pressed state to apressure released state, and part (b) of FIG. 11 is a side illustrationshowing a state of the cam member during the change of the state betweenthe heating unit and the pressing roller from the pressed state to thepressure released state.

Part (a) of FIG. 12 is a side illustration showing an engaging statebetween transmission gears in the pressure-released state between theheating unit and the pressing roller, and part (b) of FIG. 12 is a sideillustration showing a state of the cam member in the pressure-releasedstate between the heating unit and the pressing roller.

Part (a) of FIG. 13 is a side illustration showing an engaging statebetween the transmission gears during a change of a state between theheating unit and the pressing roller from the pressure released state tothe pressed state, and part (b) of FIG. 13 is a side illustrationshowing a state of the cam member during the change of the state betweenthe heating unit and the pressing roller from the pressure releasedstate to the pressed state.

Part (a) of FIG. 14 is a graph showing a relationship between a rotationangle and displacement when the cam member rotates one full turn, and(b) of FIG. 14 is a graph showing a relationship between the rotationangle and a torque on a rotation shaft of the cam member when the cammember rotates one full turn.

FIG. 15 is a graph showing a relationship between the torque on therotation shaft of the cam member and a position of a tooth of a firstgear.

Part (a) of FIG. 16 is a table showing a particular sheet of teeth a toj of a first gear 51 shown in part (a) of FIG. 10, and part (b) of FIG.16 is a table showing a particular sheet of teeth k to n of the firstgear 51 shown in part (a) of FIG. 10.

DESCRIPTION OF EMBODIMENTS

An embodiment of an image forming apparatus including a drivetransmission device according to the present invention will bespecifically described with reference to the drawings.

<Image Forming Apparatus>

First, a structure of the image forming apparatus including the drivetransmission device according to the present invention will be describedwith reference to FIG. 1. FIG. 1 is a sectional illustration showing thestructure of the image forming apparatus. An image forming apparatus 1is an example of a color image forming apparatus of anelectrophotographic type and four-drum type. At a central portion of theimage forming apparatus 1 shown in FIG. 1, an image forming portion 2 asan image forming means for forming an image on a recording material P isprovided. The image forming portion 2 includes process cartridges offour colors of yellow, magenta, cyan and black. Each of the processcartridges includes a photosensitive drum 4 as an image bearing member.

At a periphery of each photosensitive drum 4, a charging roller 5 as acharging means, a developing device 6 as a developing means, and acleaning device 7 as a cleaning means are provided. Above the respectiveprocess cartridges 3, an intermediary transfer unit 11 is provided. Theintermediary transfer unit 11 includes an intermediary transfer belt 18stretched by stretching rollers 15, 16 and 17 so as to be rotatable inthe counterclockwise direction of FIG. 1.

On an inner peripheral surface side of the intermediary transfer belt18, primary transfer rollers 19 as primary transfer means are providedopposed to the photosensitive drums 4, respectively. A primary transfernip N1 is formed between a surface of each photosensitive drum 4 and anassociated primary transfer roller 19 via the intermediary transfer belt18. A secondary transfer roller 12 as a secondary transfer means isprovided opposed to the stretching roller 16 via the intermediarytransfer belt 18. A secondary transfer nip N2 is formed between thestretching roller 16 and the secondary transfer roller 12 via theintermediary transfer belt 18. A cleaning device 26 as a cleaning meansis provided opposed to the stretching roller 15 via the intermediarytransfer belt 18.

<Image Forming Operation>

Each of the photosensitive drums 4 is rotationally driven in theclockwise direction of FIG. 1. A surface of the photosensitive drum 4 iselectrically charged uniformly by the charging roller 5. The uniformlycharged surface of the photosensitive drum 4 is irradiated with light,depending on image information, from an exposure device 27 as an imageexposure means. As a result, an electrostatic latent image is formed onthe surface of the photosensitive drum 4. The electrostatic latent imageformed on the surface of the photosensitive drum 4 is supplied withtoner (developer) of an associated color from a developing roller 30 asa developer carrying member provided in the developing device 6, andthus is developed into a toner image.

The respective color toner images formed on the surfaces of therespective photosensitive drums 4 are successively primary-transferredsuperposedly onto an outer peripheral surface of the intermediarytransfer belt 18, rotating in the counterclockwise direction of FIG. 1,under application of a primary transfer bias to the primary transferrollers 19. Residual toner remaining on the surface of eachphotosensitive drum 4 after the primary transfer is removed andcollected by the cleaning device 7.

On the other hand, the recording material P accommodated in a feedingcassette 38 is fed by a feeding roller 42 and is separated and fed oneby one by a separation roller 10. Thereafter, a leading end portion ofthe recording material P is guided by a feeding guide 44 and is abuttedagainst a nip of a registration roller pair 48 which is at rest, so thatoblique movement of the recording material P is rectified. Thereafter,the registration roller pair 48 is rotationally driven at apredetermined timing, so that the recording material P is nipped and fedby the registration roller pair 48 and then is introduced into thesecondary transfer nip N2 formed by the outer peripheral surface of theintermediary transfer belt 18 and the secondary transfer roller 12.

At the secondary transfer nip N2, a secondary transfer bias is appliedfrom an unshown secondary transfer bias voltage source to the secondarytransfer roller 12, whereby the toner images superposed and carried onthe outer peripheral surface of the intermediary transfer belt 18 aretransferred onto the recording material P. Thereafter, the recordingmaterial P is fed to a fixing unit 20 as a fixing means. The fixing unit20 includes a heating unit 31 and a pressing roller 32. The recordingmaterial P on which unfixed toner images are carried is heated andpressed during nip-feeding of the recording material P by the heatingunit 31 and the pressing roller 32, so that the toner images carried onthe recording material P are heat-fixed. Thereafter, the recordingmaterial P is nipped and fed by a feeding roller pair 55 and adischarging roller pair 53 and then is discharged onto a dischargeportion 13.

In the case where printing is carried out on double surfaces of therecording material P, in a state a trailing end portion of the recordingmaterial P with respect to a movement direction is nipped by thedischarging roller pair 53, a flapper 54 rotates about a rotation shaft54 a in the counterclockwise direction, and the discharging roller pair53 is reversely rotated, so that the recording material P is guided to afeeding path 14 for double-side printing. Then, the recording material Pis fed by feeding roller pairs 56 and 58, so that the recording materialP is turned upside down and the leading end portion of the recordingmaterial P is abutted against the nip of the registration roller pair 48which is at rest, so that oblique movement of the recording material Pis rectified.

Thereafter, the registration roller pair 48 is rotationally driven at apredetermined timing, so that the toner images are transferred onto asecond surface of the recording material P at the secondary transfer nipN2. Thereafter, the recording material P passes through the fixing unit20 and thus the toner images are fixed, and then the flapper 54 rotatesabout the rotation shaft 54 a in the clockwise direction, so that therecording material P is nipped and fed by the feeding roller pair 55 andthe discharging roller pair 53 and then is discharged onto the dischargeportion 13.

<Image Forming Apparatus Main Assembly>

As shown in FIG. 9, an apparatus main assembly (image forming apparatusmain assembly) 100 of the image forming apparatus 1 includes a secondgear 61 for transmitting drive (driving force) in engagement with afirst gear 51 provided in the fixing unit 20. When the fixing unit 20 ismounted in the apparatus main assembly 100, the first gear 51 and asmall diameter gear 61 b of the second gear 61 engage with each other.

<Mounting and Demounting Unit>

The fixing unit 20 as a mounting and demounting unit fixes the tonerimage on the recording material P. As shown in FIG. 6, the fixing unit20 includes cam members 28 and 29 capable of changing a state of theheating unit 31 as a heating member and the pressing roller 32 as apressing member, which are two members provided in the fixing unit 20,between a pressed state and a pressure-released state. The fixing unit20 further includes pressing plates 22 and 23 constituting the pressingmember is combination with the cam members 28 and 29 and includespressing springs 40. These pressing springs 40 are constituted as anurging means for urging (pressing) the heating unit 31 as the heatingmember and the pressing roller 32 as the pressing member.

Further, the fixing unit 20 includes the first gear 51 connected withthe cam members 28 and 29 via a rotation shaft 50. The first gear 51 isprovided coaxially with the cam members 28 and 29. The cam members 28and 29 rotates in interrelation with rotation of the first gear 51 viathe rotation shaft 50 and controls an urging force by the pressingsprings (urging means) 40 via the pressing plates 22 and 23 as pressingmembers. The pressing plates (pressing members) 22 and 23 press the cammembers (cams) 28 and 29.

The fixing unit 20 is constituted detachably mountable to the apparatusmain assembly 100 with respect to an arrow A direction of FIG. 2. Asshown in FIG. 3, the fixing unit 20 is constituted by including theheating unit 31, the pressing roller 32, the feeding roller pair 55, afeeding guide 49 and an outer casing 57.

<Heating Member>

As shown in FIG. 3, the heating unit 31 as the heating member includes acylindrical flexible fixing film 33 as a rotatable heating member. Theheating unit 31 further includes a guiding member 34 which has asubstantially semicircular trough shape in cross-section and which has aheat-resistant property and rigidity. Further, the heating unit 31includes a heater 35 as a heating source. The heater 35 is engaged andfixed in a groove provided along a longitudinal direction on the outerperipheral surface of the guiding member 34. The fixing film 33 isloosely fitted externally around an outer periphery of the guidingmember 34 on which the heater 35 is mounted. Further, the heating unit31 includes a pressing stay 36 having a U-shape in cross-section andrigidity. The pressing stay 36 is provided inside the guiding member 34.

As shown in FIG. 4, regulating members 37 are provided opposed tolongitudinal end portions of the fixing film 33. Each of the regulatingmembers 37 regulates a longitudinal position of the fixing film 33 andregulates a traveling locus of an outer peripheral surface or an innerperipheral surface of the fixing film 33. In this embodiment, as shownin FIG. 5, by flanges 37 a of the regulating members 37, not onlymovement of the fixing film 33 in the longitudinal direction isprevented but also the traveling locus of the fixing film 33 atlongitudinal end portions is regulated. The fixing film 33 is astructure consisting of a composite layer in which a heat-resistantresin belt or a metal belt is used as a base layer, and on an outerperipheral surface of the base layer, an elastic layer, a parting layerand the like are formed. The fixing film 33 is formed with a flexiblemember which is thin as a whole and which has high thermal conductivityand low thermal capacitance.

The heater 35 is consisting of an elongated thin linear heating memberwhich extends in the longitudinal direction perpendicular to a feedingdirection of the recording material P and which has low thermalcapacitance. The heater 35 is constituted by including a substrate of aceramic material such as aluminum nitride or alumina and an energizationheat generating layer of a silver-palladium alloy or the like formed onthe surface of the substrate. The heater 35 consisting of a ceramicheater is well known and therefore will be omitted from detaileddescription.

<Second Pressing Member>

As shown in FIG. 3, the pressing roller 32 as a second pressing memberis constituted by forming, on an outer peripheral surface of a rotationshaft 32 a consisting of a core metal, an elastic layer 32 b of asilicone rubber or the like. In order to improve non adhesiveness, on anouter peripheral surface of the elastic layer 32 b, a layer of afluorine containing resin material such as PTFE(polytetrafluoroethylene) may also be formed as a surface layer 32 c. Asanother example, a layer of a fluorine containing resin material such asPFA (tetrafluoroethylene perfluoroalkylvinyl ether copolymer) may alsobe formed as the surface layer 32 c.

As another example, a layer of a fluorine-containing resin material suchas FEP (tetrafluoroethylene-hexafluoropropylene copolymer) may also beformed as the surface layer 32 c. As shown in FIG. 4, at onelongitudinal end portion of the rotation shaft 32 a of the pressingroller 32, a driving gear 41 is provided. To the driving gear 41, arotational driving force is transmitted from a driving source providedon the apparatus main assembly 100 side. As a result, the pressingroller 32 is rotated.

As shown in FIG. 5, on an inner peripheral surface side of the fixingfilm 33, the guiding member 34, the heater 35, the pressing stay 36 andthe like are provided. The regulating member 37 is urged toward thepressing roller 32 by the pressing spring 40. As a result, the fixingfilm 33 is press contacted to the pressing roller 32 via the pressingstay 36, the guiding member 34 and the heater 35. As a result, a fixingnip N3 is formed between the outer peripheral surface of the fixing film33 and the pressing roller 32. The fixing film 33 is rotated by rotationof the pressing roller 32.

As shown in FIG. 5, the regulating members 37 are supported by sideplates 39. The recording material P passes through the fixing nip N3 ina state in which the recording material P overlaps with the fixing film33 in intimate contact with the fixing film 33. The recording material Ppasses through the fixing nip N3 while being nipped and fed in thefixing nip N3 by the outer peripheral surface of the fixing film 33 andthe pressing roller 32. In a process thereof, heat energy is impartedfrom the heater 35 to the unfixed toner image, carried on the recordingmaterial P, via the fixing film 33, so that the unfixed toner image isheated and melted and thus is heat-fixed on the recording material P.

Thereafter, the recording material P passes through the fixing nip N3and is separated from the outer peripheral surface of the fixing film 33by high stiffness of the recording material P and by the action of aseparating member 45 shown in FIG. 3. Further, the recording material Pis sent to the feeding roller pair 55 by being guided by the separatingmember 45, a feeding guide 47 provided above a recording materialfeeding path where the separating member 45 is provided, and a feedingroller 46 provided on the separating member 45. Thereafter, therecording material P is fed by the feeding roller pair 55 and thedischarging roller pair 53 and thus is discharged onto the dischargeportion 13.

<Pressing Member>

As shown in FIG. 6, both longitudinal end portions of the rotation shaft32 a of the pressing roller 32 are rotatably supported by bearingportions 21 mounted on the side plates 39 provided in the fixing unit20. The heating unit 31 as the heating member is supported by the sideplates 39 so as to be movable in a press-contact direction to thepressing roller 32. The regulating members 37 of the heating unit 31shown in FIG. 4 are pressed via the pressing plates 22 and 23 by anurging force of the pressing springs 40 as a part of the pressing membershown in FIG. 6, whereby the fixing nip N3 is formed between the heatingunit 31 and the pressing roller 32.

As shown in FIGS. 6 and 9, one end portion 22 b of each of the pressingplates 22 and 23 is inserted into and engaged with a hole 24 a providedin supporting frames 24 and 25 provided on the side plates 39. The oneend portion of pressing plate 23 and corresponding hole in thesupporting frame 25 are not shown in the figures, but, in thisembodiment, it has the same configuration as one end portion 22 b andhole 24 a. The pressing springs 40 pressing the regulating members 37shown in FIG. 4 are provided between the supporting frame 24 and thepressing plate 22 and between the supporting frame 25 and the pressingplate 23. A spacing (separating) means of the fixing nip N3 in thisembodiment includes the cam members 28 and 29 for changing pressureapplied to the fixing nip N3. The cam members 28 and 29 rotateintegrally with the first gear 51 via the rotation shaft 50 shown inFIG. 6.

At longitudinal end portions of the rotation shaft 50, the cam memberssymmetrical in shape with each other are provided. By rotation of thecam members 28 and 29, cam surfaces of the cam members 28 and 29 act onthe pressing plates 22 and 23, so that the pressure applied to thefixing nip N3 is changed. The cam members 28 and 29 acting on thepressing plates 22 and 23 are rotated by transmitting a rotationaldriving force from a motor 63 as a driving source of a drivetransmission device 60 shown in FIG. 9 to the first gear 51 provided atone longitudinal end portion of the rotation shaft 50.

As shown in FIG. 7, the cam members 28 and 29 includes the cam surfaces28 a and 29 a for controlling positions of the pressing plates 22 and23. At one longitudinal end portion of the rotation shaft 50 on whichthe cam members 28 and 29 are fixed, an unshown flag member fordetecting and controlling a state of the fixing nip N3 is provided. Bydetecting whether the flag member rotating integrally with rotation ofthe rotation shaft 50 blocks an optical path of an unshown lighttransmission sensor or permits light transmission, a pressed state and apressure released state of the fixing nip N3 are detected.

During image formation, as shown in FIG. 7, the cam members 28 and 29are held in a state in which the cam members 28 and 29 do not contactthe pressing plates 22 and 23. As a result, the heating unit 31 iscontacted to the pressing roller 32 by urging the regulating members 37by the pressing plates 22 and 23 with an urging force of the pressingsprings 40, so that the fixing nip N3 is formed.

In the case where the fixing nip N3 is required to be eliminated due tojam clearance or the like, the rotational driving force is transmittedto the first gear 51 fixed at one longitudinal end portion of therotation shaft shown in FIG. 6, so that the cam members 28 and 29 rotateintegrally with the rotation shaft. Then, as shown in FIG. 8, the camsurfaces 28 a and 29 a of the cam members 28 and 29 contact and slide onthe pressing plates 22 and 23. As a result, the pressing plates 22 and23 rotate in the counterclockwise direction of FIG. 8 about rotationcenters 22 a and 23 a, engaged with the supporting frames 24 and 25,against the urging force of the pressing springs 40.

As a result, the heating unit 31 is spaced from the pressing roller 32,so that the fixing nip N3 is eliminated. In a state in which the fixingnip N3 is eliminated, permanent deformation of the elastic layer 32 b ofthe pressing roller 32 can be suppressed. Further, the recordingmaterial P jammed in the fixing unit 20 can be easily removed.

In this embodiment, the rotational driving force is transmitted from themotor 63 shown in FIG. 9 to the first gear 51 by the drive transmissiondevice 60 when a power source of the image forming apparatus 1 is turnedoff and when a jam of the recording material P occurs. The motor 63 isdriven and controlled by an unshown CPU (central processing unit) as acontrol means. As a result, the cam members 28 and 29 are rotatedintegrally with the rotation shaft 50, so that elimination of the fixingnip N3 is carried out.

The elimination of the fixing nip N3 may also be performed byalleviating the pressure of the pressing springs 40 without completelyeliminating the pressure. This can be easily carried out byappropriately setting a shape of the cam surfaces 28 a and 29 bconsisting of outer peripheral surfaces of the cam members 28 and 29. Byappropriately setting the shape of the cam surfaces 28 a and 29 a, it ispossible to prepare various patterns different in pressure.

Further, when the cam members 28 and 29 rotate, as shown in FIG. 7, thecam surfaces 28 a and 29 a of the cam members 28 and 29 are spaced fromthe pressing plates 22 and 23. Then, by the urging force of the pressingsprings 40, the pressing plates 22 and 23 are rotated in the clockwisedirection of FIG. 7 about the rotation centers 22 a and 23 a engagedwith the supporting frames 24 and 25. As a result, the pressing plates22 and 23 urge the regulating members 37, so that the heating unit 31 iscontacted to the pressing roller 32 and thus the fixing nip N3 is in thepressed state.

<Drive Transmission Device>

FIG. 9 is a perspective illustration showing a structure of the drivetransmission device 60 for transmitting the rotational driving force tothe cam members 28 and 29. The drive transmission device 60 transmitsdrive (driving force) between the apparatus main assembly 100 and thefixing unit 20 as a mounting and demounting unit provided detachablymountable to the apparatus main assembly 100. The drive transmissiondevice 60 shown in FIG. 9 includes the second gear 61 as a driving gearand the first gear 51 as a driven (follower) gear engaging with thesecond gear 61. The second gear 61 is provided on the apparatus mainassembly 100 side and is rotated by transmitting thereto the rotationaldriving force from the motor 63 as a driving source. The first gear 51is provided on the fixing unit 20 side as shown in FIG. 6 and is rotatedin engagement with the small diameter gear 61 b of the second gear 61 asshown in FIG. 9.

On the apparatus main assembly 100 side, the second gear 61 as a drivinggear, the motor 63 and transmission gears 62 a and 62 b are provided.The transmission gear 62 b is engaged with a driving gear 63 a of themotor 63, and a large diameter gear 62 a 1 of the transmission gear 62 aconsisting of a two stage gear is engaged with the transmission gear 62b. Further, a large diameter gear 61 a of the second gear 61 consistingof a two stage gear is engaged with a small diameter gear 62 a 2 of thetransmission gear 62 a. As a result, by rotational drive of the motor63, the second gear 61 is rotated via the driving gear 63 a, thetransmission gear 62 b and the transmission gear 62 a.

As shown in FIG. 9, on the fixing unit 20 side, the first gear 51 as thedriven gear, the rotation shaft 50, the cam member 28, the pressingspring 40, the pressing plate 22 and the supporting frame 24 areprovided. Rotation of each of the first gear 51 and the cam member 28 isprevented through parallel pins 52 provided on the rotation shaft 50.The first gear 51 is provided at an engaging position with the smalldiameter gear 61 b of the second gear 61 provided on the apparatus mainassembly 100 side in a state in which the fixing unit 20 is mounted atan image forming position of the apparatus main assembly 100. By therotational driving force transmitted to the first gear 51 from thesecond gear 61 rotated by rotational drive of the motor 63, the cammember 28 is rotated via the rotation shaft.

Next, with reference to FIGS. 10 to 14, an engaging state between thesmall diameter gear 61 b of the second gear 61 and the first gear 51when the cam member 28 is rotated one full turn by drive transmissionfrom the second gear 61 as the driving gear to the first gear 51 as thedriven gear will be described. Part (a) of FIG. 10 is a sideillustration showing the engaging state between the small diameter gear61 b of the second gear 61 and the first gear 51 in a pressed statebetween the heating unit 31 and the pressing roller 32. Part (b) of FIG.10 is a side illustration showing a state of the cam member 28 in thepressed state between the heating unit 31 and the pressing roller 32.Phases of the first gear 51 and the cam member 28 shown in (a) and (b)of FIG. 10 are 0°.

Part (a) of FIG. 11 is a side illustration showing an engaging statebetween the small diameter gear 61 b of the second gear 61 and the firstgear 51 during a change of a state between the heating unit 31 and thepressing roller 32 from the pressed state shown in parts (a) and (b) ofFIG. 10 to the pressure-released state shown in parts (a) and (b) ofFIG. 12. Part (b) of FIG. 11 is a side illustration showing a state ofthe cam member 28 during the change of the state between the heatingunit 31 and the pressing roller 32 from the pressed state shown in parts(a) and (b) of FIG. 10 to the pressure-released state shown in parts (a)and (b) of FIG. 12. Phases of the first gear 51 and the cam member 28shown in parts (a) and (b) of FIG. 11 are in a state in which the firstgear 51 and the cam member 28 are integrally rotated in the clockwisedirection from a state of 0° shown in parts (a) and (b) of FIG. 10 to astate of 50° shown in parts (a) and (b) of FIG. 11.

Part (a) of FIG. 12 is a side illustration showing an engaging statebetween the small diameter gear 61 b of the second gear 61 and the firstgear 51 in the pressure-released state between the heating unit 31 andthe pressing roller 32. Part (b) of FIG. 12 is a side illustrationshowing a state of the cam member 28 in the pressure-released statebetween the heating unit 31 and the pressing roller 32. Phases of thefirst gear 51 and the cam member 28 shown in parts (a) and (b) of FIG.12 are in a state in which the first gear 51 and the cam member 28 areintegrally rotated in the clockwise direction from a state of 0° shownin parts (a) and (b) of FIG. 10 to a state of 180° shown in parts (a)and (b) of FIG. 12.

Part (a) of FIG. 13 is a side illustration showing an engaging statebetween the small diameter gear 61 b of the second gear 61 and the firstgear 51 during a change of a state between the heating unit 31 and thepressing roller 32 from the pressure-released state shown in parts (a)and (b) of FIG. 12 to the pressed state shown in parts (a) and (b) ofFIG. 10. Part (b) of FIG. 13 is a side illustration showing a state ofthe cam member 28 during the change of the state between the heatingunit 31 and the pressing roller 32 from the pressure-released stateshown in parts (a) and (b) of FIG. 12 to the pressed state shown inparts (a) and (b) of FIG. 10. Phases of the first gear 51 and the cammember 28 shown in parts (a) and (b) of FIG. 13 are in a state in whichthe first gear 51 and the cam member 28 are integrally rotated in theclockwise direction from a state of 0° shown in parts (a) and (b) ofFIG. 10 to a state of 250° shown in parts (a) and (b) of FIG. 13.

The small diameter gear 61 b of the second gear 61 provided on theapparatus main assembly 100 side includes 13 teeth and has a normalinvolute shape which is a tooth surface shape of a normal gear. On theother hand, the first gear 51 provided on the fixing unit 20 sideincludes 14 teeth (teeth a to n shown in parts (a) of FIGS. 10 to 13).

As shown in parts (a) and (b) of FIG. 13, by the cam members (cams) 28and 29 rotating integrally with the first gear 51 via the rotation shaft50, two members (the heating unit 31 and the pressing roller 32) ischanged in state from the pressure-released state to the pressed state.At that time, an urging force of the pressing springs 40 acts on the camsurfaces 28 a and 29 a of the cam members 28 and 29, so that the firstgear 51 rotates in advance in the clockwise direction of part (a) ofFIG. 13. A region where the first gear 51 rotates in advance and engageswith the small diameter gear 61 b (second gear) of the second gear 61 isdefined as a preceding rotational region R1 shown in part (a) of FIG.13.

Tooth surfaces 51 d and 51 e of four teeth k to n, of the first gear 51,corresponding to the preceding rotational region R1 have such aninvolute shape that the tooth surfaces 51 d and 51 e are subjected topositive addendum modification (increase in tooth thickness by increasein addendum circle diameter) of 0.5 mm toward an outside of the firstgear 51 with respect to a radial direction. A root circle 51 f of thefour teeth k to n corresponding to the preceding rotational region R1 inwhich the tooth surfaces 51 d and Me are subjected to positive addendummodification of 0.5 mm toward the outside of the first gear 51 withrespect to the radial direction is taken into consideration. The rootcircle 51 f of the four teeth k to n has such a shape that the fourteeth k to n are cut toward an inside of the first gear 51 with respectto the radial direction correspondingly to 0.5 mm in which the toothsurfaces 51 d and 51 e are subjected to positive addendum modificationof 0.5 mm. As a result, the first gear 51 is formed so that diameters ofroot circles 51 c and 51 f of 14 teeth a to j and k to n coincide witheach other.

A shape of 10 teeth a to j corresponding to a region other than thepreceding rotational region R1 of the first gear 51 shown in part (a) ofFIG. 13 is set as follows. A tooth surface 51 a on a side (left side ofthe tooth surface) where drive is transmitted from the small diametergear 61 b of the second gear 61 rotating in the counterclockwisedirection of part (a) of FIG. 11 has a normal involute shape which is atooth surface shape of a normal gear. On the other hand, also a toothsurface 51 b on an opposite side (right side of the tooth surface) hasthe normal involute shape which is the tooth surface shape of the normalgear, but a tooth top 51 b 1 is cut.

The teeth (first teeth) k to n of the first gear 51 have the normalinvolute shape such that the tooth top is not cut, but the root circleis cut in an amount corresponding to 0.5 mm. As a result, generation ofabutment of a tooth bottom of the small diameter gear 61 b when thesmall diameter gear 61 b of the second gear 61 engages with the teeth(first teeth) k to n of the first gear 51 is prevented. The teeth(second teeth) a to j of the first gear 51 are cut obliquely but atintermediary transfers thereof, so that free ends thereof are sharp(pointed).

As regards the first gear 51 in this embodiment, an amount of positiveaddendum modification of the plurality of teeth (second teeth) a to jshown in part (a) of FIG. 16 is 0.27 mm, and an amount of positiveaddendum modification of the plurality of teeth (first teeth) k to nshown in part (b) of FIG. 16 is 0.77 mm. For this reason, the amount ofpositive addendum modification of the plurality of teeth (first teeth) kto n shown in part (b) of FIG. 16 is set so as to be larger than theamount of positive addendum modification of the plurality of teeth(second teeth) a to j shown in part (a) of FIG. 16. As another example,in the case where the plurality of teeth (second teeth) a to j of thefirst gear 51 are normal teeth with the amount of positive addendummodification of 0 mm (i.e., are not subjected to the positive addendummodification), the plurality of teeth (first teeth) k to n of the firstgear 51 can be constituted by teeth subjected to the positive addendummodification by a predetermined amount of positive addendummodification.

Part (a) of FIG. 14 is a graph showing a relationship between a rotationangle and displacement when the cam member 28 rotates one full turn.Part (b) of FIG. 14 is a graph showing a relationship between therotation angle and a torque on the rotation shaft 50 of the cam member28. As shown in part (a) of FIG. 14, when the cam member 28 rotates to aposition of 45° from a position of the pressed state (0°), between theheating unit 31 and the pressing roller 32, shown in parts (a) and (b)of FIG. 10, the cam surface 28 a of the cam member 28 abuts against andslides on the pressing plate 22, so that displacement increases.

Thereafter, as shown in parts (a) and (b) of FIG. 11, the cam member 28rotates to a position of 50°, and then as shown in parts (a) and (b) ofFIG. 11, the cam member 28 rotates to a position of 180° where theheating unit 31 and the pressing roller 32 are in the pressure-releasedstate.

Thereafter, as shown in parts (a) and (b) of FIG. 13, the cam member 28rotates to a position of 250°. At this time, as shown in part (a) ofFIG. 14, the displacement lowers. Then, an urging force of the pressingspring 40 acts on the cam surface 28 a of the cam member 28 via thepressing plate 22, and thus tries to rotate the first gear 51 earlierthan drive transmission from the small diameter gear 61 b of the secondgear 61. As a result, as shown in part (b) of FIG. 14, a precedingrotational region R2 in which the torque on the rotation shaft 50 isnegative appears. Thereafter, the cam member 28 rotates to a position of360°, and thus returns to the position of the pressed state shown inparts (a) and (b) of FIG. 10.

Here, before the cam members 28 and 29 rotate in advance, there is aneed that the teeth (first teeth) k to n of the first gear 51 in thepreceding rotational region R1 reliably engage with associated teeth ofthe small diameter gear 61 b of the second gear 61. For this reason, thepreceding rotational region R1 of the teeth (first teeth) k to n of thefirst gear shown in FIG. 13 is set so as to be larger than the precedingrotational region R2 of the cam members 28 and 29 shown in part (b) ofFIG. 14 (R1>R2).

Further, a region (portion of the tooth n of the first gear 51) in whichthe preceding rotation of the cam members 28 and 29 ends is considered.A region in which the pressing plates 22 and 23 and the cam members 28and 29 are in non-contact with each other and before the cam members 28and 29 are rotated again by the motor 63 via the drive transmissiondevice 60 is considered. At this time, setting is made so thatengagement of the teeth (first teeth) k to n of the first gear 51 in thepreceding rotational region R1 with the teeth of the small diameter gear61 b of the second gear 61 ends before the torque on the rotation shaft50 shifts to a positive side.

The graph of the cam members 28 and 29 shown in part (a) of FIG. 14 isconsidered. A region from an angle (225°) where an outer diameter of thecam members 28 and 29 starts to lower from a maximum outer diameter(pressure released position) to an angle (322°) where the outer diameterof the cam members 28 and 29 becomes a minimum outer diameter isconsidered. The rotation angle at this time is set at a rotation anglewhere the teeth k to n of the first gear 51 are provided.

That is, in this embodiment, the rotation angle of the precedingrotational region R1 of the teeth (first teeth) k to n of the first gear51 until the first gear 51 rotates in advance and then engages with thesmall diameter gear (second gear) 61 b of the second gear 61 is 97°(=322°−225°). Depending on the shape of the cam members 28 and 29, therotation angle can also be set at less than 97° or more than 97°. Whennoise reduction or the like is taken into consideration, the precedingrotational region R1 may preferably be set from 80° to 100°.

FIG. 15 is a graph showing a relationship between the torque on therotation shaft 50 of the cam member 28 and positions of the teeth a to nof the first gear 51. As shown in parts (a) of FIGS. 10 to 13, the cammember 28 and the first gear 51 are fixed on the rotation shaft 50 byparallel pins 52. As a result, a positional relationship of the torqueon the rotation shaft 50 with the teeth a to n of the first gear 51 isshape controlled as shown in FIG. 15. The tooth surfaces 51 a on a sidewhere the teeth a to j where the tooth tops 51 b 1 of the first gear 51are cut are provided correspondingly to a range of the torque on therotation shaft 50 from 0 (zero) to positive (+). The tooth surfaces, Mdand Me of the teeth k to n of the first gear 51 subjected to thepositive addendum modification are provided correspondingly to thepreceding rotational region R1 which is a range in which the torque onthe rotation shaft 50 is negative ( ).

Next, with reference to FIGS. 11 and 13, forces generated in the firstgear 51 and the small diameter gear 61 b of the second gear 61 at eachof a position when the cam member 28 rotates 50° from the pressed state(0°) shown in part (b) of FIG. 10 and a position when the cam member 28rotates 250° from the pressed state (0°) will be described. As shown inparts (a) and (b) of FIG. 11, a position (50°) during a change from thepressed state (0°) shown in parts (a) and (b) of FIG. 10 to thepressure-released state (180°) shown in parts (a) and (b) of FIG. 12will be considered. At this time, the first gear 51 is rotationallydriven in the clockwise direction of part (a) of FIG. 11 by the smalldiameter gear 61 b of the second gear 61 rotationally driven in thecounterclockwise direction of part (a) of FIG. 11 by the motor 63 as adriving source.

At this time, drive (driving force) in an arrow B direction of part (a)of FIG. 11 is transmitted to the tooth surface Ma of the tooth d of thefirst gear 51 from a tooth surface 61 b 1 of the small diameter gear 61b of the second gear 61 rotating in the counterclockwise direction ofpart (a) of FIG. 11. Then, in the fixing unit 20, a force generates in adirection (engaging direction between the gears) in which the fixingunit 20 is drawn into the apparatus main assembly 100. As a result,engagement between the first gear 51 and the small diameter gear 61 b ofthe second gear 61 is ensured.

On the other hand, by the cam member 28, the two members (the heatingunit 31 and the pressing roller 32) are changed in state from thepressure-released state (180°) shown in parts (a) and (b) of FIG. 12 tothe pressed state (360°) shown in parts (a) and (b) of FIG. 10. At aposition (250°) during the change in state, as shown in parts (a) and(b) of FIG. 13, an urging force of the pressing spring 40 acts on thecam surface 28 a of the cam member 28 via the pressing plate 22. By thisaction, the first gear 51 is rotated in the clockwise direction of part(a) of FIG. 13 earlier than the drive transmission from the smalldiameter gear 61 b of the second gear 61. As a result, precedingrotation of the cam member 28 and the first gear 51 generates. As aresult, the driving side is momentarily changed to the first gear 51side.

Thus, the precedently rotating tooth surface Me of the tooth 1,subjected to the positive addendum modification, of the first gear 51shown in part (a) of FIG. 13 receives a reaction force in an arrow Ddirection from a tooth surface 61 b 2 of the small diameter gear 61 b ofthe second gear 61. At that time, by the reaction force, on the fixingunit 20, a force of moving the fixing unit 20 away from the apparatusmain assembly 100 acts.

When the tooth surface 51 e of the first gear 51 rotates in advance andabuts against the tooth surface 61 b 2 of the small diameter gear 61 bof the second gear (input gear) 61, noise generates. By the reactionforce received by the tooth surface 51 e during the preceding rotation,the fixing unit 20 is liable to move away from the apparatus mainassembly 100. For this reason, backlash between the first gear 51 andthe second gear (input gear) 61 is enlarged, so that impact noiseincreases.

However, in this embodiment, the tooth surfaces 51 d and Me of the teethk to n of the first gear 51 in the preceding rotational region R1 issubjected to the positive addendum modification in an amountcorresponding to backlash (0.5 mm in this embodiment) in which movementof the fixing unit 20 is received by the apparatus main assembly 100.For this reason, a width (tooth thickness) of the tooth surface Mdincreases. That is, the tooth thicknesses of the teeth (first teeth) kto n, subjected to the device addendum modification, of the first gear51 in the preceding rotational region R1 are thicker than the tooththicknesses of the teeth (second teeth) a to j, subjected to cutting ofthe tooth top 51 b 1, of the first gear 51.

The preceding rotational region R1 shown in part (a) of FIG. 13 is aregion where the first gear 51 engages with the small diameter gear(second gear) 61 b of the second gear 61 when the heating unit 31 andthe pressing roller 32 (which are the two members) are changed in statefrom the pressure-released state to the pressed state by the cam members(cams) 29 and 29. At positions corresponding to the preceding rotationalregion R1, the plurality of teeth (first teeth) k to n, subjected to thepositive addendum modification, of the first gear 51 are disposed. Atpositions corresponding to a region other than the preceding rotationalregion R1, the plurality of teeth (second teeth) a to j, subjected tocutting of the tooth top 51 b 1, of the first gear 51.

As a result, play between the tooth surface 51 d (51 e) of the teeth kto n subjected to the positive addendum modification and a tooth surface61 b 1 (61 b 2) of the small diameter gear 61 b of the second gear 61decreases. That is, backlash between each of the teeth (first teeth) kto n, subjected to the positive addendum modification, of the first gear51 in the preceding rotational region R1, and associated one of theteeth of the small diameter gear (second gear) 61 b of the second gear61 will be considered. The backlash is smaller than backlash betweeneach of the teeth (second teeth) a to j, subjected to cutting of thetooth top 51 b 1, of the first gear 51, and associated one of the teethof the small diameter gear (second gear) 61 b of the second gear 61.

As a result, even when the tooth surface 51 e of the tooth 1, subjectedto the positive addendum modification, of the first gear 51 receives thereaction force from the tooth surface 61 b 2 of the small diameter gear61 b of the second gear 61, movement of the fixing unit 20 can bereceived by the apparatus main assembly 100. As a result, engagementbetween the first gear 51 and the small diameter gear 61 b of the secondgear 61 is ensured.

A position of the tooth surface 51 d of the tooth k, subjected to thepositive addendum modification, of the first gear 51 shown in part (a)of FIG. 13 is provided in a region where a torque on the rotation shaft50 is substantially 0 (zero), not a negative (−) value, as shown in FIG.15. For this reason, the tooth k, subjected to the positive addendummodification, of the first gear 51 and the small diameter gear 61 b ofthe second gear 61 are engaged with each other before these portionsenter the preceding rotational region R1 in which the preceding rotationof the cam member 28 starts. As a result, play formed between the toothsurface 51 e of the tooth 1 and the tooth surface 61 b 2 of the smalldiameter gear 61 b of the second gear 61 is small when the cam member 28and the first gear 51 rotate in advance in the clockwise direction ofparts (a) and (b) of FIG. 13. For this reason, impact noise when thetooth surface 51 e and the tooth surface 61 b 2 collide with each othercan be reduced.

Next, a positional relationship between the first gear 51 and the smalldiameter gear 61 b of the second gear 61 when the fixing unit 20 ismounted in the apparatus main assembly 100 will be described. In a stateof the fixing unit 20 alone, the urging force of the pressing spring 40acts on the cam surface 28 a of the cam member 28 via the pressing plate22, and rotates the cam member 28 and the first gear 51 to the positionof the pressed state (0°) shown in parts (a) and (b) of FIG. 10.

For this reason, the preceding rotational region R1, of the first gear51, in which the cam member 28 and the first gear 51 rotate in advanceis prevented from being maintained at a position corresponding to thesmall diameter gear 61 b as shown in parts (a) and (b) of FIG. 13. Whenthe fixing unit 20 is mounted in an image forming position of theapparatus main assembly 100, as shown in part (a) of FIG. 10, the teetha to j, subjected to the cutting of the tooth top 51 b 1 of the firstgear 51 are in positions where the teeth engage with the small diametergear 61 b. For this reason, the tooth surfaces Md and Me of the teeth kto n, which are not subjected to the cutting of the tooth surface but issubjected to the positive addendum modification, of the first gear 51 inthe preceding rotational region R1 are prevented from contacting thetooth surfaces 61 b 1 and 61 b 2 of the small diameter gear 61 b of thesecond gear 61 as shown in part (a) of FIG. 13.

the fixing unit 20 is mounted in the image forming position of theapparatus main assembly 100, the tooth surfaces 51 a and 51 b of theteeth a to j, subjected to the cutting of the tooth top 51 b 1, of thefirst gear 51 always contact the tooth surfaces 61 b 1 and 61 b 2 of thesmall diameter gear 61 b of the second gear 61. As a result, when thefixing unit 20 is mounted in the image forming position of the apparatusmain assembly 100, there is no obstruction of mounting of the fixingunit 20 in a predetermined position of the apparatus main assembly 100by abutment between the tooth tops 51 b 1 of the first gear 51 and thesmall diameter gear 61 b of the second gear 61.

In this embodiment, there is a need to prevent mutual abutment of thetooth tops 51 b 1 of the gears engaging with each other when the fixingunit 20 is mounted and demounted. For this reason, the first gear 51 isprevented from engaging with the small diameter gear 61 b at the teeth kto n, subjected to the positive addendum modification, provided in thepreceding rotational region R1 of the first gear 51. This is, the firstgear 51 is configured so that the first gear 51 does not stop byengagement of the teeth k to n, subjected to the positive addendummodification, provided in the preceding rotational region R1, with theteeth of the small diameter gear 61 b. For that reason, phases of thecam members 28 and 29 are controlled and the teeth k to n provided inthe preceding rotational region R1 of the first gear 51 mounted on therotation shaft 50 of the cam members 28 and 29 which can be controlledare subjected to the positive addendum modification.

Incidentally, a stop position of the second gear (input gear) 61 canalso be controlled by mounting a sensor or the like so that a stop phaseof the second gear (input gear) 61 can be controlled. When such phasecontrol can be carried out, it is also possible to provide the teeth,subjected to the positive addendum modification in a regioncorresponding to the preceding rotational region of the second gear(input gear) 61. Further, it is also possible to employ the first gear51 and the second gear (input gear) 61 each including teeth whichcorrespond to an associated preceding rotational region and which aresubjected to the positive addendum modification to some degree.

Part (a) of FIG. 16 is a table showing a particular sheet of the teeth ato j of the first gear 51 shown in part (a) of FIG. 10. Part (b) of FIG.16 is a table showing a particular sheet of the teeth k to n of thefirst gear 51 shown in part (a) of FIG. 10. Backlash between each ofnormal teeth consisting of the teeth a to j, subjected to the cutting ofthe tooth top 51 b 1, of the first gear 51 and the associated one of theteeth of the small diameter gear 61 b of the second gear (input gear) 61is 0.15 mm.

As regards an amount of the positive addendum modification of the teethk to n, subjected to the positive addendum modification, of the firstgear 51 in the preceding rotational region R1, an amount of addendummodification of 0.77 mm for each of the teeth k to n, subjected to thepositive addendum modification, of the first gear 51 in the precedingrotational region R1 is taken into consideration. Further, an amount ofaddendum modification of 0.27 mm for each of the teeth a to j, subjectedto the cutting of the tooth top 51 b 1, of the first gear 51 is takeninto consideration. The amount of the positive addendum modification ofeach of the teeth k to n is 0.5 mm (=0.77 mm-0.27 mm) which is a valueobtained by subtracting 0.27 mm from 0.77 mm.

For this reason, as regards a designed value of the backlash betweeneach of the thick teeth (the teeth k to n subjected to the positiveaddendum modification) of the first gear 51 and the associated one ofthe teeth of the small diameter gear 61 b of the second gear (inputgear) 61, backlash of 0.15 mm between each of the teeth a to j of thefirst gear 51 and the associated one of the teeth of the small diametergear 61 b is taken into consideration. Further, an amount of thepositive addendum modification of 0.5 mm for each of the teeth k to n,subjected to the positive addendum modification, of the first gear 51 istaken into consideration. The designed value of the backlash is 0.35 mm(=0.15 mm 0.5 mm) which is a value obtained by subtracting 0.5 mm from0.15 mm. Here, the fixing unit 20 moves 0.5 mm away from the apparatusmain assembly 100, and therefore, actual backlash is 0.15 mm.

The teeth k to n, subjected to the positive addendum modification, ofthe first gear 51 have a normal involute shape in which the tooth top isnot cut. The root circle 51 f of each of the teeth k to n is cut.

A root circle diameter of each of all the teeth a to n of the first gear51 is 17.79 mm. Specifically, the root circle of the first gear 51 isthe same as the root circle in the case where the first gear 51 isconstituted by only the teeth (second teeth) a to j subjected to thecutting of the tooth top 51 b 1. Also as regards the root circle of eachof the teeth k to n, subjected to the positive addendum modification, ofthe first gear 51, in order to prevent root circle collision, thediameter of the root circle is set at 17.79 mm which is the samediameter as the root circle diameter of each of the teeth a to j of thefirst gear 51.

Here, the root circle diameter in the case where all the teeth of thefirst gear 51 are assumed to be constituted by only the teeth a to j iscalculated by the following formula 1 by using the number of teeth of14, a module of 1.5, an amount of addendum modification of 0.27, and adedendum coefficient of 1.25 which are shown in part (a) of FIG. 16.

Root circle diameter=14×1.5+0.27×2−1.25×1.5×2=17.79 (mm)  (formula 1)

In the formula 1, 14×1.5=21 (mm) represents a reference pitch circlediameter of the teeth a to j of the first gear 51 shown in part (a) ofFIG. 16. To this value, a value (0.27×2) which is twice the addendummodification amount of 0.27 mm of each of the teeth a to j is added, sothat a pitch circle diameter of the teeth a to j for which the addendummodification is taken into consideration is acquired. From this pitchcircle diameter, a value (1.25×1.5×2) which is obtained by multiplyingthe dedendum coefficient of 1.25 by the module of 1.5 and by 2 issubtracted, so that the root circle diameter of the teeth a to j can beacquired.

<Measurement of Tooth Thickness>

As regards measurement of a tooth thickness of each of the teeth (secondteeth) a to j and each of the teeth (first teeth) k to n of the firstgear 51, a base tangent length (base thickness of teeth) is determinedso that the teeth and their associated teeth contact each othersubstantially in the neighborhood of a meshing tooth depth (height). Asregards the teeth (second teeth) a to j of the first gear 51 shown inpart (a) of FIG. 16, the base thickness of teeth (the thickness of twoteeth as measured over the two teeth) when the number of teeth in asector span is “2” is “7.121 mm”. On the other hand, as regards theteeth (first teeth) k to n of the first gear 51 shown in part (b) ofFIG. 16, the base thickness of teeth (the thickness of three teeth asmeasured over the three teeth) when the number of teeth in a sector spanis “3” is “11.891 mm”.

By using a known calculating formula in combination with the “basethickness of teeth”, the “number of teeth in a sector span”, the“module” and the “pressure angle”, a tooth teeth T2 of each of the teeth(second teeth) a to j is calculated from the associated “base thicknessof teeth” in accordance with a formula 2 shown below. Further, a toothteeth T1 of each of the teeth (first teeth) k to n is calculated fromthe associated “base thickness of teeth” in accordance with a formula 3shown below. Incidentally, in the terms of the following formulas 2 and3, “π×1.5×cos 20°” represents a base (normal) pitch (mm).

$\begin{matrix}{{T\; 2} = {{7.121 - {\left( {2 - 1} \right) \times \pi \times 1.5 \times \cos \; 20{^\circ}}} = {2.69\mspace{14mu} ({mm})}}} & \left( {{formula}\mspace{14mu} 2} \right) \\{{T\; 2} = {{11.891 - {\left( {3 - 1} \right) \times \pi \times 1.5 \times \cos \; 20{^\circ}}} = {3.03\mspace{14mu} ({mm})}}} & \left( {{formula}\mspace{14mu} 3} \right)\end{matrix}$

From the above formulas 2 and 3, the tooth thickness T1 of each of theteeth (first teeth) k to n of the first gear 51 is thicker than thetooth thickness T2 of each of the teeth (second teeth) a to j of thefirst gear 51.

The first gear 51 of this embodiment includes the teeth (second teeth) ato j and the teeth (first teeth) k to n, but the root circle diameter ofthe first gear 51 is designed so as to be 17.79 (mm). That is, the rootcircle diameter is the same as the root circle diameter in the casewhere all the teeth of the first gear 51 are assumed to be constitutedby only the teeth (second teeth) a to j.

Incidentally, in a region other than the preceding rotational region R1of the first gear 51, not only the teeth (second teeth) a to j subjectedto the cutting of the tooth top 51 b 1 but also teeth having the toothtop similar to the tooth top of the teeth (first teeth) k to n subjectedto the positive addendum modification may exist in mixture.

Other Embodiments

In the above-described embodiment, as the drive transmission device 60between the fixing unit 20 and the apparatus main assembly 100, anexample using the first gear 51 on the fixing unit 20 side and the smalldiameter gear 61 b of the second gear 61 on the apparatus main assembly100 side was described. However, the drive transmission device 60 isalso applicable to a connecting portion between the apparatus mainassembly 100 and, as another unit, an intermediary transfer unit 11which is an intermediary transfer member.

Further, an example in which the cam member 28 and the first gear 51which are provided in the fixing unit 20 are provided on a commonrotation shaft 50 was described. As another example, the case where anunshown cam gear is provided on the rotation shaft 50 on which the cammember 28 is provided and the first gear 51 is provided on a rotationshaft different from the rotation shaft 50 will be considered. Further,between the cam gear and the first gear 51, a follower (driven) gear mayalso be provided. In this case, a gear ratio among the cam gear, thefollower gear and the first gear 51 may also be a gear ratio such thatspeeds of the gears are not reduced so as to coincide with the phase ofthe cam member 28 or may also be a gear ratio such that the numbers ofteeth of these gears provide an integral multiple thereof. The firstgear 51 is engaged with the small diameter gear 61 b of the second gear61 provided on the apparatus main assembly 100 side.

According to the present invention, an engaging performance of the gearsfor transmitting the drive (driving force) to the cam capable ofchanging the pressed state between the two members.

While the present invention has been described with reference toexemplary embodiments, it is to be understood that the invention is notlimited to the disclosed exemplary embodiments. The scope of thefollowing claims is to be accorded the broadest interpretation so as toencompass all such modifications and equivalent structures andfunctions.

This application claims the benefit of Japanese Patent Applications Nos.2017-095484 filed on May 12, 2017 and 2018-027517 filed on Feb. 20,2018, which are hereby incorporated by reference herein in theirentirety.

1. A drive transmission device comprising: a first gear; a camconfigured to rotate in interrelation with rotation of said first gearand capable of changing a state of two members between a first state inwhich the two members are biased with a first pressure and a secondstate in which the two members are biased with a second pressure smallerthan the first pressure; a pressing member configured to press said cam;and a second gear configured to transmit a driving force to said firstgear in engagement with said first gear, wherein at least one of saidfirst gear and said second gear includes (i) a plurality of first teethprovided at positions corresponding to an engaging region in which saidfirst gear is engaged with said second gear when the state of the twomembers is changed from the second state to the first state and (ii) aplurality of second teeth provided at positions corresponding to aregion other than said engaging region, and wherein tooth thicknesses ofsaid first teeth are thicker than tooth thicknesses of said secondteeth. 2.-20. (canceled)
 21. A drive transmission device according toclaim 1, wherein a positive amount of addendum modification of saidfirst teeth is larger than a positive amount of addendum modification ofsaid second teeth.
 22. A drive transmission device according to claim 1,wherein said first gear includes a root circle, the root circle of saidfirst gear being the same as a root circle if said first gear wasconstituted only by said second teeth.
 23. A drive transmission deviceaccording to claim 1, wherein at least one of said first gear and saidsecond gear has an involute shape.
 24. A drive transmission deviceaccording to claim 1, wherein said engaging region is a region in whichsaid first gear rotates in advance and then engages with said secondgear when the state of the two members is changed from the second stateto the first state by said cam.
 25. A drive transmission deviceaccording to claim 24, wherein a rotation angle of said engaging regionuntil said first gear rotates in advance and then engages with saidsecond gear is 80° to 100°.
 26. A drive transmission device according toclaim 1, wherein said first gear is provided coaxially with said cam.27. A drive transmission device according to claim 1, wherein when saidsecond teeth are normal teeth, and said first teeth are teeth subjectedto positive addendum modification.
 28. A drive transmission devicecomprising: a first gear; a cam configured to rotate in interrelationwith rotation of said first gear and capable of changing a state of twomembers between a first state in which the two members are biased with afirst pressure and a second state in which two members are biased with asecond pressure smaller than the first pressure; a pressing memberconfigured to press said cam; and a second gear configured to transmit adriving force to said first gear in engagement with said first gear,wherein at least one of said first gear and said second gear includes(i) a plurality of first teeth provided at positions corresponding to anengaging region in which said first gear is engaged with said secondgear when the state of the two members is changed from the second stateto the first state and (ii) a plurality of second teeth provided atpositions corresponding to a region other than said engaging region, andwherein backlash between one of said first teeth and an associated toothof said second gear is smaller than backlash between one of said secondteeth and an associated tooth of said second gear.
 29. A drivetransmission device according to claim 28, wherein a positive amount ofaddendum modification of said first teeth is larger than a positiveamount of addendum modification of said second teeth.
 30. A drivetransmission device according to claim 28, wherein said first gearincludes a root circle, the root circle of said first gear being thesame as a root circle if said first gear was constituted only by saidsecond teeth.
 31. A drive transmission device according to claim 28,wherein at least one of said first gear and said second gear has aninvolute shape.
 32. A drive transmission device according to claim 28,wherein said engaging region is a region in which said first gearrotates in advance and then engages with said second gear when the stateof the two members are changed from the second state to the first stateby said cam.
 33. A drive transmission device according to claim 32,wherein a rotation angle of said engaging region until said first gearrotates in advance and then engages with said second gear is 80° to100°.
 34. An image forming apparatus including a mounting and demountingunit detachably mountable to a main assembly of said image formingapparatus, wherein said mounting and demounting unit includes, a firstgear, a cam configured to rotate in interrelation with rotation of saidfirst gear and capable of changing a state of two members between afirst state in which the two members are biased with a first pressureand a second state in which the two members are biased with a secondpressure smaller than the first pressure, and a pressing memberconfigured to press said cam; and wherein said main assembly includes asecond gear configured to transmit a driving force to said first gear inengagement with said first gear, wherein at least one of said first gearand said second gear includes (i) a plurality of first teeth provided atpositions corresponding to an engaging region in which said first gearis engaged with said second gear when the state of the two members ischanged from the second state to the first state and (ii) a plurality ofsecond teeth provided at positions corresponding to a region other thansaid engaging region, and wherein tooth thicknesses of said first teethare thicker than tooth thicknesses of said second teeth.
 35. An imageforming apparatus according to claim 34, wherein when said first teethreceive a reaction force from teeth of said second gear, said firstteeth of said first gear are subjected to positive addendum modificationcorresponding to backlash in which movement of said mounting anddemounting unit is received by said main assembly.
 36. An image formingapparatus according to claim 34, wherein at least one of said first gearand said second gear has an involute shape.
 37. An image formingapparatus according to claim 34, wherein said mounting and demountingunit is a fixing unit.
 38. An image forming apparatus according to claim34, wherein said engaging region is a region in which said first gearrotates in advance and then engages with said second gear when the stateof the two members are changed from the second state to the first stateby said cam.
 39. An image forming apparatus including a mounting anddemounting unit detachably mountable to a main assembly of said imageforming apparatus, wherein said mounting and demounting unit includes, afirst gear, a cam configured to rotate in interrelation with rotation ofsaid first gear and capable of changing a state of two members between afirst state in which the two members are biased with a first pressureand a second state in which two members are biased with a secondpressure smaller than the first pressure, and a pressing memberconfigured to press said cam, and wherein said main assembly includes asecond gear configured to transmit a driving force to said first gear inengagement with said first gear, wherein at least one of said first gearand said second gear includes (i) a plurality of first teeth provided atpositions corresponding to an engaging region in which said first gearis engaged with said second gear when the state of the two members ischanged from the second state to the first state and (ii) a plurality ofsecond teeth provided at positions corresponding to a region other thansaid engaging region, and wherein backlash between one of said firstteeth and an associated tooth of said second gear is smaller thanbacklash between one of said second teeth and an associated tooth ofsaid second gear.